Comparison of external load compensation during rhythmic arm movements andrhythmic jaw movements in humans

Experiments were performed on human elbow flexor and extensor muscles and j aw-opening and -closing muscles to observe the effect on rhythmic movements of sudden loading. The load was provided by an electromagnetic device, whi ch simulated the appearance of a smoothly increasing spring-like load. The responses to this loading were compared in jaw and elbow movements and betw een expected and unexpected disturbances. All muscles showed electromyograp hic responses to unexpected perturbations, with latencies of similar to 65 ms in the arm muscles and 25 ms in the jaw. When loading was predictable, a nticipatory responses started in arm muscles similar to 200 ms before and i n jaw muscles 100 ms before the onset of loading. The reflex responses rela tive to the anticipatory responses were smaller for the arm muscles than fo r the jaw muscles. The reflex responses in the arm muscles were the same wi th unexpected and expected perturbations, whereas anticipation increased th e reflex responses in the jaw muscles. Biceps brachii and triceps brachii s howed similar sensory-induced responses and similar anticipatory responses. Jaw muscles differed, however, in that the reflex response was stronger in masseter than in digastric. It was concluded that reflex responses in the arm muscles cannot overcome the loading of the arm adequately, which is com pensated by a large centrally programmed response when loading is predictab le. The jaw muscles, particularly the jaw-closing muscles, tend to respond mainly through reflex loops, even when loading of the jaw is anticipated. T he differences between the responses of the arm and the jaw muscles may be related to physical differences. For example, the jaw was decelerated more strongly by the load than the heavier arm. The jaw was decelerated strongly but briefly, <30 ms during jaw closing, indicating that muscle force incre ased before the onset of reflex activity. Apparently, the force-velocity pr operties of the jaw muscles have a stabilizing effect on the jaw and have t his effect before sensory induced responses occur. The symmetrical response s in biceps and triceps indicate similar motor control of both arm muscles. The differences in reflex activity between masseter and digastric muscle i ndicate fundamental differences in sensory feedback to the jaw-closing musc le and jaw-opening muscle.